The disclosure of Japanese Patent Application No. 2000-332546 filed on Oct. 31, 2000 including the specification, drawings, and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to a device for controlling the idle speed of an internal combustion engine such as a gasoline engine or the like. More particularly, the invention relates to a device for maintaining an idle speed in the case of fluctuations in external load. The invention also relates to a method of controlling the idle speed.
2. Description of Related Art
For starting an internal combustion engine such as a gasoline engine, it is required not only to be supplied with fuel but also to be forcefully rotated by an external force. Accordingly, even if a vehicle equipped with the engine requires no power, for example, because it is stopped, the internal combustion engine continues to rotate in preparation for subsequent takeoff. This is an idling state in which the engine speed (idle speed) is set as a lowest possible speed within such an extent that the engine does not come into a stall (i.e. within such an extent that engine stall does not occur). To be more precise, the idle speed is maintained by setting a target speed in accordance with the state of warm-up, the state of a load ascribable to auxiliaries, and so on and subjecting the amount of intake air to feedback control on the basis of a difference between the target speed and an actual speed.
Feedback control is performed with a view to changing the amount of intake air in response to a change in actual speed and generation of a difference between the actual speed and the target speed. Therefore, such a speed as will not cause engine stall despite a temporary drop in speed is set as a target speed during idling. For this reason, the constant idle speed is set as a speed slightly higher than a lowest speed capable of maintaining autonomous rotation. Because the amount of fuel consumption increases correspondingly, there is room for an improvement in fuel consumption.
The idle speed is set as a relatively high speed as described above with the intention of preventing the engine from coming into a stall due to a temporary drop in engine speed. Thus, once the idle speed is stabilized, it can be further reduced to the extent of achieving an improvement in fuel consumption. The change in idle speed is mainly caused by fluctuations in external load ascribable to the auxiliaries or the like. Hence, in general, various control operations corresponding to fluctuations in external load are performed for the purpose of stabilizing the idle speed.
That is, an internal combustion engine mounted in a vehicle is connected to auxiliaries such as a hydraulic pump of a power steering device, a compressor for air-conditioning, and so on. Therefore, if these auxiliaries are operated, the internal combustion engine may be exposed to a load and drop in speed even during idling. Thus, if such a load ascribable to the auxiliaries arises, the amount of intake air is increased in accordance with the load so as to maintain the idle speed. Such correction of the load ascribable to the auxiliaries is realized not by feedback control but by feedforward control.
In the case where the amount of intake air is increased or reduced due to an increase or decrease in the external load, the space extending from a throttle valve or an ISC (idle speed control) valve to an intake port of each cylinder occupies a large volume even if the opening of the valve is changed. Thus, the amount of air actually sucked into each cylinder and consumed for combustion changes with a predetermined delay with respect to a change in opening of the valve. Namely, even if the opening of the valve is changed due to an increase in the external load, the amount of intake air increases with a delay so that the speed of the internal combustion engine drops temporarily. On the contrary, in the case where the opening of the valve is reduced due to a decrease in external load, the amount of intake air decreases with a delay with respect to the decrease in external load, so that the output of the internal combustion engine becomes relatively large as compared with the external load. As a result, the speed of the internal combustion engine increases temporarily. In such a case where the amount of intake air is increased or reduced on the basis of an external load (load ascribable to the auxiliaries), the speed of the internal combustion engine becomes unstable as a result of an unavoidable delay.
In order to prevent the speed of the internal combustion engine from increasing or decreasing due to a delay of change in the amount of intake air, the invention disclosed in Japanese Patent Application Laid-Open No. 7-63149 is designed to change the amount of intake air in accordance with changes in external load while performing correction so as to advance or retard the ignition timing in accordance with actual changes in the amount of intake air, thus redressing a delay of changes in the output torque of the internal combustion engine and preventing the speed of the internal combustion engine from increasing or decreasing temporarily. To be more precise, estimation of a required torque is made from an increase in the amount of intake air in the case of fluctuations in external load. Then, calculation of a correction amount of the ignition timing is made in accordance with the estimated torque.
As disclosed in the aforementioned publication, if the ignition timing is subjected to correction for advancement so as to compensate for the torque in response to generation of an external load or an increase in external load, it is necessary to ensure a certain width of advancement in advance. Thus, the ignition timing needs to be retarded when no load is being applied. This may lead to a deterioration in fuel consumption during idling. Further, since estimation of a load torque is made and calculation of a correction amount of the ignition timing is then made on the basis of the result of estimation, the actual amount of advancement or retardation of the ignition timing to be corrected may become excessive or insufficient. This may constitute an obstacle to stabilization of the speed of the internal combustion engine.
In particular, the output torque of the internal combustion engine changes depending on the speed even on the condition that the amount of intake air be the same. Hence, simple control for achieving a target torque demonstrates a control sensitivity that varies in accordance with the engine speed. During idling in which a change in torque causes an immediate change in speed, there occurs an increased error in correction. Consequently, control of the engine speed suffers the phenomenon of hunting, so that the engine speed becomes unstable.
Further, in view of a control delay of the amount of intake air, it is also possible to further correct the amount of air instead of correcting the ignition timing as described above. However, the correction does not reflect the actual amount of air in the intake pipe. Therefore, the amount of correction becomes excessive if the external load is turned on and off repeatedly. This may cause unstableness in the speed of the internal combustion engine.
The invention has been made as a solution to the aforementioned technical problems. It is an object of the invention to provide a device capable of suppressing changes in idle speed resulting from fluctuations in external load and further stabilizing the idle speed.
In order to achieve the above-mentioned object, the invention is designed to make a determination on a transient state of the engine speed by estimating a speed of the internal combustion engine in the case where the amount of intake air is corrected in response to a change in external load, and to further reduce the amount of intake air or correct the ignition timing on the basis of the result of determination.
An idle speed control device and a method of controlling an idle speed of an internal combustion engine according to a first aspect of the invention are designed to perform control of an idle speed of the internal combustion engine by at least one of an amount of intake air and an ignition timing during application of an external load. It is preferable that calculation of a target amount of intake air corrected in response to fluctuations in external load in an idling state of the engine be made, that estimation of a transient speed of the internal combustion engine be made in the case where the amount of intake air is changed to the target amount, and that on the basis of a difference between the estimated speed and the actual speed of the internal combustion engine at that moment, at least one of the amount of intake air and the ignition timing is corrected in such a manner as to reduce the difference. Thus, in the case of fluctuations in external load, the amount of intake air corresponding to the external load is set. In a transient state where the engine assumes an operational state corresponding to the amount of intake air, the amount of intake air or the ignition timing is corrected. Thus, the speed of the internal combustion engine is controlled so as to be maintained at the estimated speed, namely, the speed to be achieved.
As a result, it is possible to suppress fluctuations in output torque of the internal combustion engine resulting from a delay of change in the amount of intake air based on the external load or to suppress changes in speed of the internal combustion engine associated with fluctuations in external load, so that the idle speed of the internal combustion engine can be stabilized.
Further, it is also possible to estimate a transient speed of the internal combustion engine on the basis of the target amount of air, the actual amount of intake air at that moment, and a target idle speed.
Further, it is also possible to estimate a transient speed of the internal combustion engine on the basis of a target torque corresponding to an external load and an actual amount of intake air at that moment.
Thus, if the speed for achieving the target torque corresponding to the external load is estimated, and if the amount of intake air or the ignition timing is corrected such that the actual speed becomes equal to the estimated speed, the output torque of the internal combustion engine is unlikely to become excessive or insufficient even in the case of fluctuations in external load. Thus, it is possible to stabilize the idle speed.
Furthermore, in the case where the output torque of the internal combustion engine is reduced, the ignition timing may be controlled in such a manner as to be retarded.
This construction is designed such that the output torque of the internal combustion engine is reduced due to retardation control of the ignition timing, thus making it possible to improve the control response of the output torque and to further stabilize the idle speed.
Further, an idle speed control device according to a second aspect of the invention is designed to control an idle speed of an internal combustion engine by at least one of an amount of intake air and an ignition timing during application of an external load. It is preferable that calculation of a target amount of air corrected on the basis of fluctuations in external load in an idling state be made, that estimation of a transient speed of the internal combustion engine be made in the case where the amount of intake air is changed to the target amount of air, and that the ignition timing be corrected in such a manner as to suppress an increase in the transient speed of the internal combustion engine.
In this case, it is possible to obtain a relation among the engine speed, the amount of air, and the correction amount of the ignition timing through an experiment conducted during actual operation of the engine. Because the idle speed is controlled on the basis of the experimental data, the precision in correcting the ignition timing is enhanced. As a result, it becomes possible to inhibit or prevent the engine speed from fluctuating.